Latent heat and storms

When energy is used to heat something up, the temperature does not increase smoothly as the energy is put in. Most significantly, this is because causing matter to change states takes energy in itself, above and beyond the energy that goes into warming. Imagine a big block of ice at 0°C. A lot of energy has to go into it before it becomes a pool of water at 0°C. The same is true for turning 100°C water into 100°C steam. Latent heat has been discussed here before.

Because of climate change, the overall trend in global air temperatures is going upward. As anyone who has visited a steam room or had a camera fog up when coming inside on a cold day knows implicitly, warmer air can hold more water. As well as being an important feedback effect (since water vapour is a greenhouse gas), warmer more air-laden water contains more of the latent heat that provides the energy for thunderstorms, tornadoes, and hurricanes. The increase in the average amount of latent heat in a body of air increases the probable strength of future storms, a fact that becomes especially worrisome when you acknowledge how the damage caused by storms increases in a non-linear way. Winds that are 10% faster have a third more destructive potential.

The extra water in the air will also increase the quantity of precipitation and the likelihood of floods. Furthermore, melting ice sheets will cool sea water, increasing the temperature differential between the equatorial and polar regions. This will increase the strength of mid-latitude cyclones, as air currents cooled by melting ice sheets (latent heat, again) collide with ever-warmer masses of air, containing ever-more water. The level of melting in the ice sheets is already significant enough to measure using sensitive gravitational data from satellites like GRACE. Greenland is losing about 100 cubic kilometres of ice per year, while West Antarctica is losing it at a somewhat smaller rate. The ‘wet’ process of ice sheet disintegration suggests that the rate of ice loss could increase dramatically, one the ice sheets are pushed past a critical point by warming.

GRACE is the first Earth-monitoring mission in the history of space flight whose key measurement is not derived from electromagnetic waves either reflected off, emitted by, or transmitted through Earth’s surface and/or atmosphere. Instead, the mission uses a microwave ranging system to accurately measure changes in the speed and distance between two identical spacecraft flying in a polar orbit about 220 kilometers (137 miles) apart, 500 kilometers (311 miles) above Earth. The ranging system is so sensitive it can detect separation changes as small as 10 microns—about one-tenth the width of a human hair over a distance of 220 kilometers.

As the twin GRACE satellites circle the globe 16 times a day, they sense minute variations in Earth’s gravitational pull. When the first satellite passes over a region of slightly stronger gravity, a gravity anomaly, it is pulled slightly ahead of the trailing satellite. This causes the distance between the satellites to increase. The first spacecraft then passes the anomaly, and slows down again; meanwhile the following spacecraft accelerates, then decelerates over the same point.

By measuring the constantly changing distance between the two satellites and combining that data with precise positioning measurements from Global Positioning System (GPS) instruments, scientists can construct a detailed map of Earth’s gravity.

The two satellites (nicknamed “Tom” and “Jerry”) constantly maintain a two-way microwave-ranging link between them. Fine distance measurements are made by comparing frequency shifts of the link. As a cross-check, the vehicles measure their own movements using accelerometers. All of this information is then downloaded to ground stations. To establish baseline positions and fulfill housekeeping functions, the satellites also use star cameras, magnetometers, and GPS receivers. The GRACE vehicles also have optical corner reflectors to enable laser ranging from ground stations, bridging the range between spacecraft positions and Doppler ranges.

“Weaver points out that the 2007 IPCC report was, in fact, conservative with its conclusions. At the time it didn’t have access to more accurate satellite data from NASA’s Gravity Recovery and Climate Experiment (GRACE) mission.

The GRACE data and the Marshall and Barber studies are some of the more recent developments. They reinforce hundreds, even thousands of climate-change studies from the past two decades that form pieces of this complex puzzle. But there are enough pieces in place that we’re starting to see an image, and it doesn’t look good.”